Temperatures averaged over the earth’s surface have increased by about one degree since the industrial revolution and this increase can largely be attributed to emissions of well-mixed greenhouse gases and other forcings[1] from human activity like aerosol and ozone emissions and land-use changes. Globally averaged temperatures are the clearest indicator of the fact that the climate is changing but people do not experience averaged temperatures. People experience weather and are in particular impacted by extreme weather events.

Extreme weather in a warming world

In a warming world, we expect the risk for some kinds of extreme weather events occurring to increase and other, like cold spells to decrease. With increasing temperatures in most parts of the world, we expect more and hotter heat waves. At the same time, warmer air can hold more water and thus we expect more extreme rainfall. And sure enough, whenever an extreme weather event happens anywhere in the world people are asking the question about the possible role of climate change. However, the effect of climate change on extreme weather is not uniform across different regions and also varies between seasons and timescales. Furthermore, climate change does not only affect temperature but also the atmospheric circulation. This means the typical track of rainstorms or high-pressure systems can change and thus either counteract or amplify the effects from temperature changes. Individual weather events are ultimately unique and always caused by a combination of different factors including local variability in the weather, conditions of the land surface and its feedbacks with the atmosphere, large-scale patterns of ocean temperatures and changes in the external drivers (e.g. greenhouse gases, aerosols). Therefore it cannot be said that an extreme weather event was caused by climate change. However, we can now estimate whether and to what extent the likelihood or intensity of an extreme event occurring has changed due to human-induced climate change.

Attribution of extreme weather events

Extreme weather events are by definition rare. Thus, in the same way, as it is not possible to determine from throwing a dice a couple of times whether or not it is loaded we cannot say from a short observational weather record whether or not the number of a certain type of extreme events has increased. However, in some regions long, high-quality records exist and in many cases, it is possible to create a large record of simulated weather and extreme weather events with state of the art climate models and statistical methods. This allows scientists to estimate what is possible weather in the world we live in and to quantify what the likelihood of an observed extreme weather event is. Was it a 1 in 100-year event? Or a 1 in 10-year event? Having determined the likelihood of the event in today’s climate, i.e. with climate change, we can then ask what the likelihood of the event would have been in a world without human-induced climate change. While we do not have observations of the world that might have been without climate change we do know very well how much CO2 and other greenhouse gases we have emitted. This enables us to simulate possible weather in the world that might have been without human-induced climate change in the same way as we simulate possible weather events today. It is important to assess the skill of models in simulating this “world that might have been”, just as it is important to assess the skill of weather forecast models. Only where models have sufficient skill can reliable event attribution assessments be made.

Comparing these two likelihoods then allows quantifying the role of climate change. For example, a heat record of 40°C somewhere in Europe might now be expected to occur on average every 10 years but would have been a much rarer event of 1 in 100 years in a world without climate change. We can then say that climate change has made the event 10times more likely. Similarly, if we find that a 1 in 10-year event in the world without climate change would be 38°C to 2°C lower than in today’s climate,we would lead to the conclusion that climate change contributed 2°C to the magnitude of the record. It is important to highlight that not all extreme weather events are being made more likely due to climate change and for every attribution study there are always 4 possible outcomes: (1) the event has been made more likely, (2) the event has been made less likely, (3) there is no detectable change in the likelihood of the event, (4) with current tools and understanding it is not possible to make an attribution statement.

The ability of state-of-the-art climate models to simulate weather events varies strongly between different types of events and different regions which directly relates to the confidence in attribution statements. While we are very confident that the risk of a European heatwave like the one observed in 2003 has at least doubled due to climate change we can currently not attribute small-scale events like e.g., flash floods, hail storms, or tornadoes. However with better climate models and increasing numbers of scientists working on this question and scrutinizing each other’s work our methods are constantly improving.

[1] Greenhouse gases are not the only emissions originating from human activity but the burning of fossil fuels and changes in land use also leads to the emission of aerosol particles (e.g. sulfates, soot, mineral dust) that influence the climate in various ways. Agents that influence the climate system like greenhouse gases or aerosols but also changes in the sun’s orbit are called forcings.

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